Full Citation: Linden, V.M.G., Grass, I., Joubert, E., Tscharntke, T., Weier, S.M., and Taylor, P.J. 2019. Ecosystem services and disservices by birds, bats, and monkey change with macadamia landscape heterogeneity. Journal of Applied Ecology, 56: 2069-2078. https://doi.org/10.1111/1365-2664.13424
Plant it, and they will come.
Planting a garden is all fun and games until the rabbits come around and eat all your veggies. In our backyard gardens, crop loss from animals is merely unfortunate, without any dire consequences. However, for commercial farmers, crop loss from animals can threaten their entire livelihood. To combat crop loss, the logical and cost effective solution for many farmers is to cut down the native vegetation surrounding the farm where these animals reside. If there are no forests for the animals, there is no crop loss, right?
Well, the solution isn’t as clear-cut as that. Lack of native habitat means no mammalian predators (e.g. monkeys) to eat the crops. However, it also means there is no habitat for other species that may actually help farmers. Ecosystem services is a term that refers to benefits that humans freely receive from nature doing what it does. One ecosystem service farmers may receive is pest control. Animals such as birds and bats often consume the insect pests responsible for drastically reducing crop yields. When farmers destroy natural habitat, they not only destroy habitat for mammalian predators eating their crops, but they also destroy habitat for the birds and bats that help with pest management.
Do services outweigh disservices?
There is clearly a trade-off involved with having natural habitat next to farmland. Do the ecosystem services provided by birds and bats outweigh the disservices from crop predators, or is it the other way around? This topic has received very little attention, which was a big reason that a research team led by Valerie M. G. Linden set out to quantify this tradeoff. From 2015-2018, Linden and her team ran a series of experiments on macadamia orchards in South Africa. Macadamia farmers in South Africa commonly suffer crop losses due to raids by the vervet monkey. To rid these monkeys, farmers destroy adjacent patches of natural vegetation. However, several species of birds and bats have been documented to prey on some of the most problematic insect pests in macadamia orchards.
The experimental setup consisted of four treatments. The “full” enclosure involved cages being placed around macadamia trees at all times, which excluded birds, bats, and monkeys. The “day” treatment involved the cages being closed only during the day, allowing bats and nocturnal birds access to the trees. For the “night” treatment, the cages were closed only at night, which allowed monkeys and birds to access the trees during the day. Finally, there was the “control” treatment, where cages where left open all the time. The researchers also compared the effects of adjacent landscape. At each farm, the researchers employed cages adjacent to human-modified landscapes, which included crops or roads, and natural landscapes, which consisted of any natural vegetation. In total, six farms were sampled, with each farm receiving all four treatments for both landscape types. To quantitatively compare the treatments, the researchers calculated income per hectare as a measure of yield.
Cutting forests = cutting profits.
When the researchers compared results across treatments and habitat, they found striking differences in yield. In general, yield was 19% lower in the natural controls (open cages next to forested areas) than the human-modified landscape setting (open cages next to crops and roads). Within the human-modified landscape setting, the full enclosure, which excluded bats and birds (monkeys were completely absent from the human-modified landscape settings), there was a 60% decrease in yield compared to the control, suggesting that bats and birds can access human-modified settings from adjacent forests and can provide innumerable benefits. In natural landscapes, where monkeys were present, there was no significant difference between yield from the full and control. However, with the day enclosure, which was closed during the day to keep away monkeys but open at night to allow bats and nocturnal birds access to the trees, there was a 26% increase in yield. These results translate into an economic impact of a nearly 5000 USD/hectare income loss when bats and birds are excluded, compared to a loss of approximately 1600 USD/hectare from monkey raiding.
From these results, the researchers concluded that the benefits of biocontrol from birds and bats outweighed the losses from monkeys. In both landscape settings, the exclusion of bats and birds led to a decrease in yield, while the exclusion of monkeys only resulted in yield gains close to natural vegetation. When birds, bats, and monkeys were all excluded, there were still losses in yield due to the lack of pest control. Thus, there appears to be much benefit to maintaining natural vegetation around farms. However, the visible damages to crops from monkeys may still drive farmers to remove native vegetation to keep them away. Regardless, farmers can invite more birds and bats to their land by setting up artificial nesting sites. This may be the most cost-effective method farmers can use. While this was just one study in a very specific context, the results are promising, and should stimulate further research in a variety of ecological contexts. But, when it comes to birds and bats in macadamia orchards, there’s no monkeying around about how big of a help they are.